When hosting the 2010 G8 summit of major economies, the Canadian prime minister, Stephen Harper, called for an "accountability summit" to hold the G8 responsible for the promises that it made over the years. So let's make our own account of how the G8 did. The answer, alas, is a failing grade. The G8 this year illustrates the difference between photo-ops and serious global governance.

Of all of the G8's promises over the years, the most important was made to the world's poorest people at the 2005 G8 Gleneagles summit in Scotland. The G8 promised that, by this year, it would increase annual development assistance to the world's poor by $50bn relative to 2004. Half of the increase, or $25bn per year, would go to Africa.

The G8 fell far short of this goal, especially with respect to Africa. Total aid went up by around $40bn rather than $50bn, and aid to Africa rose by $10bn-$15bn per year, rather than $25bn. The properly measured shortfall is even greater, because the promises that were made in 2005 should be adjusted for inflation. Re-stating those commitments in real terms, total aid should have risen by around $60bn, and aid to Africa should have risen by around $30bn.

In effect, the G8 fulfilled only half of its promise to Africa – roughly $15bn in increased aid rather than $30bn. Much of the overall G8 increase in aid went to Iraq and Afghanistan, as part of the US-led war effort, rather than to Africa. Among G8 countries, only the UK is making a bold effort to increase its overall aid budget and direct a significant portion to Africa.

Since the G8 was off track in its aid commitments for many years, I long wondered what the G8 would say in 2010, when the commitments actually fell due. In fact, the G8 displayed two approaches. First, in an "accountability report" issued before the summit, the G8 stated the 2005 commitments in current dollars rather than in inflation-adjusted dollars, in order to minimise the size of the reported shortfall.

Second, the G8 summit communique simply did not mention the unmet commitments at all. In other words, the G8 accountability principle became: if the G8 fails to meet an important target, stop mentioning the target – a cynical stance, especially at a summit heralded for "accountability".

The G8 did not fail because of the current financial crisis. Even before the crisis, the G8 countries were not taking serious steps to meet their pledges to Africa. This year, despite massive budget crisis, the UK government has heroically honoured its aid commitments, showing that other countries could have done so if they had tried.

But isn't this what politicians like to do – smile for the cameras, and then fail to honour their promises? I would say that the situation is far more serious than that.

First, the Gleneagles commitments might be mere words to politicians in the rich world, but they are matters of life and death for the world's poor. If Africa had another $15bn-$20bn per year in development aid in 2010, as promised, with the amounts rising over future years (also as promised), millions of children would be spared an agonising death from preventable diseases, and tens of millions of children would be able to get an education.

Second, the emptiness of G8 leaders' words puts the world at risk. The G8 leaders promised last year to fight hunger with $22bn in new funds, but so far they are not delivering. They promised to fight climate change with $30bn of new emergency funds, but so far they are not delivering. My own country, the US, shows the largest gap between promises and reality.

Hosting this year's G8 summit reportedly cost Canada a fortune, despite the absence of any significant results. The estimated cost of hosting the G8 leaders for a day and a half, followed by the G20 leaders for a day and a half, reportedly came to more than $1bn. This is essentially the same amount that the G8 leaders pledged to give each year to the world's poorest countries to support maternal and child health.

It is absurd and troubling to spend $1bn on three days of meetings under any circumstances (since there are much cheaper ways to have such meetings and much better uses for the money). But it is tragic to spend so much money and then accomplish next to nothing in terms of concrete results and honest accountability.

There are three lessons to be drawn from this sorry episode. First, the G8 as a group should be brought to an end. The G20, which includes developing countries as well as rich countries, should take over.

Second, any future promises made by the G20 should be accompanied by a clear and transparent accounting of what each country will do, and when. The world needs true accountability, not empty words about accountability. Every G20 promise should spell out the specific actions and commitments of each country, as well as the overall promise of the group.

Third, the world's leaders should recognise that commitments to fight poverty, hunger, disease, and climate change are life-and-death issues that require professional management for serious implementation.

The G20 meets later this year in South Korea, a country that has emerged from poverty and hunger over the past 50 years. South Korea understands the utter seriousness of the global development agenda, and the poorest countries' needs. Our best hope is that South Korea will succeed as the next host country, picking up where Canada has fallen far short.